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The following plot show the time evolution of peak positions for B, C, N and O peaks: 
All four peaks show linear decrease with very close relative slope:
- B: 1.49+-0.05% per year
- C: 1.60+-0.04% per year
- N: 1.50+-0.06% per year
- O: 1.46+-0.03% per year
This signal decrease is probably explained by radiation damage to CsI crystals and has the value consistent with pre-launch expectations ~1% per year
I've done the proton calibration of individual crystals for the same time periods and compared the results with the initial on-orbit calibration done on June 28, 2008 and used of reconstruction so far.
The results are shown on the following plot: